1. Field of the Invention
The present invention relates to an automatic transmission.
2. Discussion of the Background
There is a known automatic transmission capable of forward 8-speed transmission by using a first planetary gear system for input, second and third planetary gear systems for transmission, and six engagement mechanisms (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-273768).
According to Japanese Unexamined Patent Application Publication No. 2005-273768, the first planetary gear system is a double-pinion planetary gear system and includes a first sun gear, a first ring gear, and a first carrier, which supports paired first pinions engaging with each other and engaging with the first sun gear and the first ring gear in such manner that the first pinions freely rotate and orbit. The first planetary gear system is also known as a “plus planetary gear system” or a “positive planetary gear system” because, when the carrier is fixed, the sun gear and the ring gear rotate in the same direction. When the ring gear is fixed, the sun gear and the carrier rotate in the different directions.
In the first planetary gear system, the first sun gear is a fixed element, which is fixed to the transmission case, the first carrier is an input element connected to an input shaft, and the first ring gear is an output element, which reduces the rotational speed of the first carrier and outputs the reduced rotational force.
The two planetary gear systems for transmission are each a Ravigneaux-type planetary gear system, which includes a second sun gear, a third sun gear, a second ring gear integrated with a third ring gear, and a second carrier, which supports paired second pinions engaging with each other, one of which engaging with the second sun gear and the second ring gear and the other engaging with third sun gear and the third ring gear, in such manner that the second pinions freely rotate and orbit.
According to a collinear diagram of the Ravigneaux-type planetary gear system (in which the ratios of the relative rotational speeds of different elements are represented by straight lines), if the elements are defined as a first rotary element, a second rotary element, a third rotary element, and a fourth rotary element on the basis of the distances thereof corresponding to the gear ratios, the first rotary element is the second sun gear, the second rotary element is the second carrier integrated with the third carrier, the third rotary element is the second ring gear integrated with the third ring gear, and the fourth rotary element is the third sun gear.
The automatic transmission includes engagement mechanisms: a first wet multi-disc clutch, which freely connects and disconnects the first ring gear, which is the output element of the first planetary gear system, and the fourth rotary element, which is the third sun gear; a second wet multi-disc clutch, which freely connects and disconnects the input shaft and the second rotary element, which is the second carrier; a third wet multi-disc clutch, which freely connects and disconnects the first ring gear, which is the output element, and the first rotary element, which is the second sun gear; a fourth wet multi-disc clutch, which freely connects and disconnects the first carrier, which is the input element, and the first rotary element, which is second sun gear; and a first brake, which freely fixes and detaches the first rotary element, which is the second sun gear, to the transmission case; and a second brake, which freely fixes and detaches the second rotary element, which is the second carrier, to the transmission box.
With the configuration described above, first gear is established by engaging the first wet multi-disc clutch and the second brake; second gear is established by engaging the first wet multi-disc clutch and the first brake; third gear is established by engaging the first wet multi-disc clutch and the third wet multi-disc clutch; and fourth gear is established by engaging the first wet multi-disc clutch and the fourth wet multi-disc clutch.
Fifth gear is established by engaging the first wet multi-disc clutch and the second wet multi-disc clutch; sixth gear is established by engaging the second wet multi-disc clutch and the fourth wet multi-disc clutch; seventh gear is established by engaging the second wet multi-disc clutch and the third wet multi-disc clutch; and eighth gear is established by engaging the second wet multi-disc clutch and the first brake.
A known automatic transmission has eight columns aligned with the axis of the input shaft. Specifically, in order from the torque converter side, the first column is the fourth clutch and the first brake; the second column is the first planetary gear system; the third column is the first clutch; the fourth column is the third clutch (although the third clutch is illustrated in the same column as the first planetary gear system in a skeleton diagram, actually, a piston and an oil passage for the third clutch are interposed between the first clutch and the output gear); the fifth column is the output gear; the sixth column is the second planetary gear system; the seventh column is the third planetary gear system; and the eighth column is the second clutch and the second brake.